Plant biomass as a source of energy production can include agricultural and forestry products, associated by-products and waste, municipal solid waste, and industrial waste. In addition, over 50 million acres in the United States are currently available for biomass production, and there are a number of terrestrial and aquatic crops grown solely as a source for biomass (A Wiselogel, et al. Biomass feedstocks resources and composition. In C E Wyman, ed. Handbook on Bioethanol: Production and Utilization. Washington, D.C.: Taylor & Francis, 1996, pp 105-118). Biofuels produced from biomass include ethanol, methanol, biodiesel, and additives for reformulated gasoline. Biofuels are desirable because they add little, if any, net carbon dioxide to the atmosphere and because they greatly reduce ozone formation and carbon monoxide emissions as compared to the environmental output of conventional fuels. (P Bergeron. Environmental impacts of bioethanol. In C E Wyman, ed. Handbook on Bioethanol: Production and Utilization. Washington, D.C.: Taylor & Francis, 1996, pp 90-103).
Plant biomass is the most abundant source of carbohydrate in the world due to the lignocellulosic materials composing the cell walls of all higher plants. Plant cell walls are divided into two sections, the primary and the secondary cell walls. The primary cell wall, which provides structure for expanding cells (and hence changes as the cell grows), is composed of three major polysaccharides and one group of glycoproteins. The predominant polysaccharide, and most abundant source of carbohydrates, is cellulose, while hemicellulose and pectin are also found in abundance. Cellulose is a linear beta-(1,4)-D-glucan and comprises 20% to 30% of the primary cell wall by weight. The secondary cell wall, which is produced after the cell has completed growing, also contains polysaccharides and is strengthened through polymeric lignin covalently cross-linked to hemicellulose.
Carbohydrates, and cellulose in particular can be converted to sugars by well-known methods including acid and enzymatic hydrolysis. Enzymatic hydrolysis of cellulose requires the processing of biomass to reduce size and facilitate subsequent handling. Mild acid treatment is then used to hydrolyze part or all of the hemicellulose content of the feedstock. Finally, cellulose is converted to ethanol through the concerted action of cellulases and saccharolytic fermentation (simultaneous saccharification fermentation (SSF)). The SSF process, using the yeast Saccharomyces cerevisiae for example, is often incomplete, as it does not utilize the entire sugar content of the plant biomass, namely the hemicellulose fraction.
The cost of producing ethanol from biomass can be divided into three areas of expenditure: pretreatment costs, fermentation costs, and other costs. Pretreatment costs include biomass milling, pretreatment reagents, equipment maintenance, power and water, and waste neutralization and disposal. The fermentation costs can include enzymes, nutrient supplements, yeast, maintenance and scale-up, and waste disposal. Other costs include biomass purchase, transportation and storage, plant labor, plant utilities, ethanol distillation, and administration (which may include technology-use licenses). One of the major expenses incurred in SSF is the cost of the enzymes, as about one kilogram of cellulase is required to fully digest 50 kilograms of cellulose. Economical production of cellulase is also compounded by factors such as the relatively slow gowth rates of cellulase-producing organisms, levels of cellulase expression, and the tendency of enzyme-dependent processes to partially or completely inactivate enzymes due to conditions such as elevated temperature, acidity, proteolytic degradation, and solvent degradation.
Enzymatic degradation of cellulose requires the coordinate action of at least three different types of cellulases. Such enzymes are given an Enzyme Commission (EC) designation according to the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (Eur. J. Biochem. 264: 607-609 and 610-650, 1999). Endo-beta-(1,4)-glucanases (EC 3.2.1.4) cleave the cellulose strand randomly along its length, thus generating new chain ends. Exo-beta-(1,4)-glucanases (EC 3.2.1.91) are processive enzymes and cleave cellobiosyl units (beta-(1,4)-glucose dimers) from free ends of cellulose strands. Lastly, beta-D-glucosidases (cellobiases: EC 3.2.1.21) hydrolyze cellobiose to glucose. All three of these general activities are required for efficient and complete hydrolysis of a polymer such as cellulose to a subunit, such as the simple sugar, glucose.
Highly thermostable enzymes have been isolated from the cellulolytic thermophile Acidothermus cellulolyticus gen. nov., sp. nov., a bacterium originally isolated from decaying wood in an acidic, thermal pool at Yellowstone National Park. A. Mohagheghi et al., (1986) Int. J. Systematic Bacteriology, 36(3): 435-443. One cellulase enzyme produced by this organism, the endoglucanase EI, is known to display maximal activity at 75° C. to 83° C. M.P. Tucker et al. (1989), Bio/Technology, 7(8): 817-820. E1 endoglucanase has been described in U.S. Pat. No. 5,275,944. The A. cellulolyticus E1 endoglucanase is an active cellulase; in combination with the exocellulase CBH I from Trichoderma reesei, E1 gives a high level of saccharification and contributes to a degree of synergism. Baker J O et al. (1994), Appl. Biochem. Biotechnol., 45/46: 245-256. The gene coding EI catalytic and carbohydrate binding domains and linker peptide were described in U.S. Pat. No. 5,536,655. E1 has also been expressed as a stable, active enzyme from a wide variety of hosts, including E. coli, Streptomyces lividans, Pichia pastoris, cotton, tobacco, and Arabidopsis (Dai Z, Hooker B S, Anderson D B, Thomas S R. Transgenic Res. 2000 February; 9(l):43-54).
There is a need within the art to generate alternative cellulase enzymes capable of assisting in the commercial-scale processing of cellulose to sugar for use in biofuel production. Against this backdrop the present invention has been developed. The potential exists for the successful, commercial-scale expression of heterologous cellulase polypeptides, and in particular novel cellulase polypeptides with or without any one or more desirable properties such as thermal tolerance, and partial or complete resistance to extreme pH inactivation, proteolytic inactivation, solvent inactivation, chaotropic agent inactivation, oxidizing agent inactivation, and detergent inactivation. Such expression can occur in fungi, bacteria, and other hosts.